1. Technical Field
The presently disclosed subject matter relates to a white LED light source device and an LED backlight using the same. In particular, the disclosed subject matter relates to a white LED light source device and an LED backlight using the white LED light source device which can emit white light having a spectrum containing three primary color wavelength components of red, green, and blue light by additive color mixture of light emitted from two different LED lamps with different color tones.
2. Description of the Related Art
A semiconductor light-emitting device (for example, LED device) can emit light with a sharp spectrum, which can be recognized by human beings as light with color tone corresponding to a peak wavelength λp (being the wavelength having a maximum luminous intensity). Namely, the light emitted from such a light-emitting device is not white light (natural light) including wavelengths in a range from ultraviolet to infrared wavelength and including the visible range, like sunlight. Instead, the light is a particular tone light intrinsic to the LED device in accordance with the LED devices' material, composition, structure, and the like.
Heretofore, several methods have been proposed to obtain white light using LED devices as a light source. One example of such methods is to utilize a phosphor material, or a wavelength conversion material, with an LED device. This method can use a principle in that a phosphor material is irradiated with light to be excited to thereby emit light which has a longer wavelength than that of the excitation light.
For example, a yellow phosphor material is excited by blue light (being light having a peak wavelength in the wavelength region of blue color), thereby wavelength-converting the blue light into complementary colored light, being yellow light or yellowish green light. Accordingly, when using a blue LED device and a yellow phosphor material, part of blue light emitted from the LED device can excite the yellow phosphor material to allow the phosphor material to emit yellow or yellowish green light. The resulting yellow or yellowish green light is combined with the rest of the blue light from the blue LED device for additive color mixing, thereby generating white light (see, for example, the conventional art disclosed in Japanese Patent No. 2927279).
Another conventional method utilizes two different phosphor materials including a green phosphor material, which can be excited by blue light to wavelength-convert the blue light to green light, and a red phosphor material, which can be excited by the blue light to wavelength-convert it to red light. Namely, in this instance, part of blue light emitted from the blue LED device can excite the green phosphor material to allow it to emit green light. Furthermore, part of blue light emitted from the blue LED device can excite the red phosphor material to allow it to emit red light. The resulting green light and red light are mixed with the rest of the blue light for additive color mixing, thereby generating white light (see, for example, the conventional art disclosed in Japanese Patent Application Laid-Open No. 2002-060747 and corresponding U.S. Pat. No. 6,686,691B1).
Some methods have been proposed in which phosphor materials are not used. In one such method, a red LED device which can emit red light, a green LED device which can emit green light, and a blue LED device which can emit blue light are used to simultaneously emit three colored light. By separately controlling the intensities of the red light, green light, and blue light from the respective LED devices, a white light having a desired tone can be generated by additive color mixing (see, for example, the conventional art disclosed in Japanese Patent Application Laid-Open No. 2003-100108 and corresponding U.S. Pat. No. 6,834,981B2).
In the conventional techniques described above, the method for generating white light by the combination of the blue LED device and the yellow phosphor material can provide a high utilization efficiency of blue light from the LED device as well as a luminous efficiency nearly equal to that of a common cold cathode fluorescent lamp. However, the produced white light may contain only limited amounts of red and green wavelength components, and therefore, is pseudo white light. When a light source device utilizing such a method is applied as a light source for an LCD backlight, the color reproduction characteristics of the LCD may deteriorate (for example, in the case as shown in FIG. 1).
The method for obtaining white light by the combination of a blue LED device with green and red phosphor materials can produce light containing three primary color wavelength components of blue light, green light, and red light. Accordingly, when this method is adopted as a light source for an LCD backlight, a wide range of color reproduction characteristics for the LCD can be ensured. Furthermore, since the light source includes only one type of LED device, e.g., a blue LED device, wavelength shift due to generated heat or change over time may be reduced. This can prevent the color tone of produced white light from varying. In this instance, however, green light emitted from the green phosphor material may be absorbed (re-wavelength-converted) by the red phosphor material. As a result of this, blue light from the blue LED device may not be effectively utilized and its utilization efficiency as well as luminous efficiency may deteriorate (for example, the case as shown in FIG. 2).
The method for producing white light by the combination of three primary colored LED devices, or red, green, and blue LED devices, can achieve a wide range of color reproduction characteristics, and over 100% in terms of NTSC ratio. In this instance, the respective LED devices may be formed of different materials and compositions. For example, the red LED device may be made of an AlGaInP-based material whereas the blue and green LED devices may be made of an InGaN-based material. Accordingly, each LED device has inherent temperature characteristics and deterioration characteristics. This means that the wavelength shift due to generated heat and change over time may be varied device to device, and the color tone variation of produced white light may become remarkable. Specifically, the luminous efficiency of the red LED device may deteriorate due to temperature rise with time, and therefore, the color tone of produced white light may be shifted toward blue. In addition to this, service life of the red LED device is shorter than other LED devices, and therefore, the color component thereof may be reduced with time, thereby shifting the color tone of produced white light toward blue.
By the way, when three primary colored LED lamps are used to produce white light with favorable color mixture, the lamps must be well designed in terms of arrangement, pitch therebetween, distance between both the light emitting surfaces and the surface to be irradiated, and other factors. Accordingly, if a certain distance is required or desired from the light emitting surface of the LED lamps to a surface to be irradiated as a light path to ensure optical performance, miniaturization and thinning of the apparatus incorporating the LED lamps may be hampered. When three types of LED lamps (including a red LED lamp, a green LED lamp, and a blue LED lamp) are arranged in line to produce white light with sufficient luminous intensity and uniform color tone for use as a light source of an LCD backlight, for example, it may be required to satisfy the condition of L≧P×1.5 wherein L is the distance between the light emitting surfaces of the LED lamps and the surface to be irradiated, and P is the pitch between adjacent LED lamps (see, for example, the configuration as shown in FIG. 3, which will be described later).